Field of the Invention The invention relates to a device for converting an electric current comprising at least one phase module having an AC voltage connection and at least one DC voltage connection, connected to a DC voltage intermediate circuit, and comprising at least one energy store, wherein a phase module branch is formed between each DC voltage connection and each AC voltage connection, and wherein each phase module branch has a series circuit formed by submodules each having at least one power semiconductor, wherein semiconductor protective means connected in parallel with one of the power semiconductors of each submodule and a control unit for driving the semiconductor protective means are provided and the energy store or energy stores is/are designed for supplying energy to the control unit.
The invention furthermore relates to a method for limiting damage to a converter having power semiconductors, said converter being connected to at least one further converter or at least one electrical machine via a DC voltage intermediate circuit affected by a short circuit.
The device of the generic type is already known from DE 103 23 220 A1, which discloses a so-called multiterminal converter for connection to an energy-feeding three-phase supply system. The previously known converter has phase modules, the number of which corresponds to the number of supply system phases to be connected. Each phase module has one AC voltage connection and two DC voltage connections, wherein the DC voltage connections of the phase modules are connected to a DC voltage intermediate circuit. Phase module branches extend between each AC voltage connection and each DC voltage connection, wherein each phase module branch comprises a series circuit of submodules. Each submodule has a dedicated energy store connected in parallel with a power semiconductor circuit. The power semiconductor circuit has turn-off power semiconductors, such as IGBTs, GTOs or the like, with each of which a respective freewheeling diode is reverse-connected in parallel. If a short circuit is present in the DC voltage circuit, the energy stores of the respective submodule are discharged. In order to avoid destruction of the power semiconductors of the submodules, each power semiconductor jeopardized by a short circuit in the DC voltage circuit has connected in parallel with it a semiconductor protective means, e.g. a thyristor, which is triggered in the case of a short circuit and subsequently carries a large part of the short-circuit current. What is disadvantageous about the previously known solution is that the triggering unit that triggers the thyristor is supplied with energy by the energy store of the submodules. Although this obviates a separate energy supply of the triggering units, the energy stores of the submodules are not yet charged when the converter is connected to the three-phase system, with the result that a triggering of the thyristors is impossible. Consequently, when the supply system is connected in, destruction of the power semiconductors of the previously known converter cannot be ruled out if a short circuit is present in the DC voltage circuit.